This invention relates generally to production of consolidated bodies from metal powders and more particularly to a process and apparatus for simultaneously compacting and surface treating metal powder to form a metallic body which is sufficiently dense for subsequent mechanical hot working.
Techniques for producing consolidated bodies, including billets, bars, etc., from elemental and prealloyed metal powders, such as atomized high speed tool alloys, have been known for some time. Processes and apparatus for carrying out the same which have been known for producing consolidated metal powder bodies include charging a container with the metal powder, heating the charged container to a preselected temperature at which at least some sintering will occur, and then consolidating the metal powder to eliminate, or at least minimize, porosity. Some processes include chemical or thermal pretreatment of the metal powder to enhance its sinterability by removing oxides on the surfaces of the metal powder particles. The consolidation step is accomplished by means of either mechanical or isostatic compaction. Although most of the known processes, and their corresponding apparatus, for producing consolidated metal powder bodies have provided satisfactory products, each has certain technical and/or economic drawbacks.
A published article, L. Mott, Progress Report on Hot Forging Prealloyed Metal Powders, Precision Metal Molding, Oct. 1952, relates to a process of forming metal objects from prealloyed metal powders, primarily tool steels. The process includes the steps of compacting the prealloyed powder in a shaped container, sintering the compacted powder in a controlled atmosphere to prevent decarburization, followed by hot coining. Mott discloses that the compacted powder may be sintered in an atmosphere of hydrogen or hydrogen bearing compounds. Mott describes the production of a fully dense metal body having a fine metallurgical microstructure. According to Mott, because of the way they are made, each powder particle prior to consolidation is in the ideal state for providing a uniform carbide distribution in that each is a supersaturated solid solution. Those particles when consolidated at a temperature below the formation temperature of the massive carbide can then be heat treated to precipitate carbon as small substantially spheroidal carbides, uniformly distributed throughout the consolidated body, the size of the precipitated carbide particles being a function of the precipitating temperature.
Comstock et al, British Patent No. 781,083, Aug. 14, 1957, relates to a method which includes compacting a prealloyed powder into a shaped form at room temperature, heating the compacted powder in a reducing atmosphere to a preselected temperature, and then hot coining to final shape and density.
Reen, U.S. Pat. No. 3,150,444, Sept. 29, 1964, relates to a method of producing an alloy steel body from prealloyed metal powder. The method includes the steps of forming a fine particle alloy steel powder, green compacting the powder such as by rolling, sintering the green compact in the presence of a carbonaceous reducing atmosphere, and then mechanically working the sintered body until it achieves a denseness of at least 90% of that attained by the same alloy in the normally cast and wrought form. Reen , U.S. Pat. No. 3,150,444 indicates that the carbonaceous reducing atmosphere may consist of at least 0.1% by volume hydrocarbon gas and the balance essentially a nonhydrocarbon reducing gas such as hydrogen. According to Reen, U.S. Pat. No. 3,150,444, high speed tool steel bodies produced following the process exhibit an even distribution of small carbides which had long been known to be desirable in such tool steel members.
Reen, U.S. Pat. No. 3,244,506, Apr. 5, 1966, relates to a method of producing cutting tool alloy bodies from a prealloyed metal powder. The method includes forming a fine metal powder, deoxidizing the metal powder by exposing it to hydrogen. gas, packing the powder into a mild steel tube, and evacuating and sealing the powder-filled tube. The sealed tube is heated to about 2150 F. and then extruded through a conventional extrusion die. Reen, U.S. Pat. No. 3,244,506 states that extrusion pressures used in the examples range from 2100-2700 psi. The resulting metal alloy body is stated to have a denseness substantially equivalent to the alloy in its cast state.
Pfeiler et al., U.S. Pat. No. 3,419,935, Jan. 7, 1969, and Havel, U.S. Pat. No. Re. 28,301, Jan. 14, 1975, a reissue of U.S. Pat. No. 3,622,313, Nov. 23, 1971, relate to hot isostatic pressing (HIP), another well known process, by which encapsulated metal powder is heated and compacted by a fluid, usually a gas, under a pressure of at least 500 psi while it is at a selected consolidation temperature. HIP units are, however, very expensive to construct and install because they must withstand high pressure.
DiGiambattista, U.S. Pat. No. 3,704,508, Dec. 5, 1972, relates to consolidation by atmospheric pressure in which the metal powder is first treated with a sintering activation agent and then sealed in an evacuated glass container or mold. The container is then heated in a standard air atmosphere furnace to sinter the metal powder. The sintering activation agent is intended to accelerate sintering by chemically combining with metal oxides on the powder particle surfaces to form compounds, such as borates which do not inhibit bonding. Black et al., U.S. Pat. No. 4,227,927, Oct. 14, 1980, also relates to consolidation by atmospheric pressure. Consolidation by atmospheric pressure is typically performed at temperatures close to the solidus of the particular alloy since such temperatures tend to promote densification. However, when a glass container is used, the glass container softens and shrinks as consolidation occurs during sintering. Therefore glass containers must be supported so the mass will not lose shape during sintering. Graphite or clay-graphite crucibles, like the glass containers they are used to support, are readily broken when being handled or moved, thereby adding to the cost of the process.
Holtz, Jr., U.S. Pat. No. 3,746,518, July 17, 1973, and U.S. Pat. No. 4,469,514, Sept. 4, 1984, relate to a method for producing iron, chromium, nickel, and/or cobalt based metal powder bodies by a process which includes forming a prealloyed metal powder of the desired composition in which carbides are said to be submicroscopic and consolidating by hot working to form a body said to be substantially fully dense and containing uniformly distributed carbides less than 3 microns in major dimension. The only discernible difference between the Holtz, Jr. process and the prior Mott and Comstock processes appears to reside in Holtz, Jr.'s assertions regarding carbide size and distribution.
Ayers, U.S. Pat. No. 3,834,004, Sept. 10, 1974, relates to a method of producing billets from powdered tool steel. The Ayers process includes a thermal treatment of heating encapsulated powder to a temperature in the range of 1700 to 2250 F., followed by mechanically hot working the heated powder. The intermediate product produced by the Ayers thermal treatment step is less than 90% of theoretical density. Such high porosity makes it necessary to mechanically hot work the intermediate product in such a way that excessive interpowder-particle strains do not occur, otherwise cracking of the metal powder body and/or tearing of the encapsulating canister may result. Consequently, consolidation must be carried out as a series of relatively light mechanical hot working and reheating steps which tend to prolong the process.
Bergman et al., U.S. Pat. No. 3,893,852, July 8, 1975 relates to the introduction of a non-reactive gas into a canister containing the metal powder charge in order to increase the heat transfer to the metal powder as it is being heated in a HIP process. The gas is used during a thermal pretreatment step in which the encapsulated metal powder is heated in a conventional furnace prior to sealing of the container and insertion into a pressurized furnace or HIP unit.
Smith, Jr. et al., U.S. Pat. No. 4,268,708, May 19, 1981, relates to a process and apparatus in which the metal powder is subjected to liquid phase sintering in a vessel to provide a workpiece substantially free of porosity. The workpiece is then subjected to an isostatic gas pressure of approximately 15,000 psi in the same vessel for a preselected time period in order to further consolidate it. The combination of liquid phase sintering and hot isostatic pressing functions in one vessel however, renders it more complex and time consuming than conventional HIP units.
Cold mechanical or isostatic pressing are techniques utilizing high pressure to compact a metal powder into a predetermined shape at ambient temperature. Although the process is carried out at room temperature, extremely high pressures, for example, at least about 15,000 psi for isostatic pressing and at least about 30,000 psi for mechanical pressing, are required to obtain the desired compaction. The resulting consolidated powder body is significantly less than fully dense, however, being densified to only 60-80% of theoretical density. Such low denseness is not sufficient to permit mechanical hot working and further consolidation as by sintering is required in order to achieve the necessary denseness in the metal powder body.
The foregoing processes have left much to be desired. The processes which have hitherto used mechanical cold or hot consolidation have required unduly repetitious handling and working of the bodies. The processes which rely on cold or hot isostatic pressing require relatively high pressure vessels which are expensive and inherently dangerous to operate. Additionally, cold isostatically consolidated shapes require an additional step of sintering before they can be mechanically hot worked.